Method and furnace for sintering

ABSTRACT

A method and furnace are disclosed for continuous sintering powdered metal products. The furnace includes a high temperature heating zone through which the powdered metal products pass for sintering. The high temperature heating zone includes a thermally stable medium preferably alumina sand, in which the sintered product is carried through the furnace. The invention provides a sintering furnace which is capable of achieving and maintaining higher temperatures in continuous operation than previously possible for commercially operated furnaces.

United States Patent 1 1 Griffin 1 1 Mar. 27, 1973 54 METHOD AND FURNACE FOR 3,068,091 12/1962 Kirkland ..266/24 SINTERING 3,175,889 3/1965 Titus et a1 ....26 24 3,375,099 3/1968 Marshall 2266/24 [76] Inventor: John F. Griffin, 121 Carey Avenue, 3,585,023 vnaty 8L" 75/36 Mendel, 3,407,059 10/1968 Blaha ..75/53 [22] Filed: May 20, 1970 Primary Examiner-Gerald A. Dost [21] Appl' 38937 AttorneyDouglas W. Wyatt [52] US. Cl. ..266/20, 75/200, 266/24 [57] ABSTRACT [51] Int. Cl ..F27b 21/00 [58] Field of Search 75/200, 2, 221 22-3 22 5 A method and furnace are disclosed for continuous 75/227; 266/2 R, 5 R, 5 E, 5 C, 5 T, 27, 24,

sintering powdered metal products. The furnace includes a high temperature heating zone through which the powdered metal products pass for sintering. The high temperature heating zone includes a thermally ,stable medium preferably alumina sand, in which the sintered product is carried through the furnace. The invention provides a sintering furnace which is capable of achieving and maintaining higher temperatures in continuous operation than previously possible for commercially operated furnaces.

4 Claims, 2 Drawing Figures References Cited UNITED STATES PATENTS 2/1923 Giesecke ..266/25 12/1949 Rennie ..266/24 12/1954 Davis ..266/25 5/1956 Comley ..266/5 R 7/1956 Pistorius et al.... ..266/24 1/1961 North et a1. ..7S/223 Z-i J4 Patented March 27, 1973 INVENTOR JOHN F. GK/FF/ ATTORNE 5 METHOD AND FURNACE FOR SINTERING BACKGROUND OF THE INVENTION Powdered metallurgy involves the pressing of powdered metal and the subsequent heating or sintering of the pressed metal to obtain an integral durable product. As a consequence of improved metallurgy and bonding techniques, the quality and strength of powdered metal products has been improved to a point where powdered metal parts are replacing machine parts such as drive gears which formerly required several machining operations. Through powdered metal technology it is possible in one step to produce a gear or other part directly from raw material.

After pressing, the powdered metal parts are introduced into a sintering furnace where the powdered metal parts are heated to improve quality and strength. According to present practice, conventional continuous sintering furnaces include a conventional horizontal conveyor for moving the powdered metal products through a high temperature zone. Because of the limitations in the conveyor design and materials, maximum temperatures are limited to approximately.

2,l50 Farenheit in presently used conventional sintering furnaces.

SUMMARY OF THE INVENTION The present invention provides a sintering method and a preferred furnace for carrying out the method.

The sintering method relates to sintering the pressed powdered metal parts in a thermally stable carrying medium. In carrying out the method, the metal parts are preheated to about one-half the final sintering temperature and are then mixed or distributed through the carrying medium for sintering at an elevated temperature. The carrying medium preferably is alumina sand and after sintering the metal parts are removed while the alumina sand may be reused. Of course other suitable carrying medium may be used.

The present invention also provides a high temperature sintering furnace which makes possible and sintering of powdered metal products on a continuous commercial basis at temperatures considerably in excess of those presently possible with existing conventional continuous sintering furnaces. The furnace according to the present invention, includes a preheating zone where the pressed powdered metal products are initially heated. Thereafter, themetal products are introduced into a high temperature zone. The powdered metal products are carried through the high temperature zone in a thermally stable free flowing medium which preferably is alumina sand. The high temperature furnace is advantageously arranged vertically to receive the powdered metal products mixed with the alumina sand. The alumina and the powdered metal products pass through the high temperature zone and receive heat "from a suitable source for the purpose of sintering the powdered metal products. The alumina functions as a conveying medium for carrying the powdered metal products through the high temperature furnace. 7

Flow of the carrying medium and the powdered metal products is controlled by a take-off conveyor located at the bottom of the vertical high temperature furnace. That is to say, the rate of flow of medium through the furnace is effectively controlled by the rate at which the take-off conveyor removes the carrying medium and sintered product at the bottom of the vertical furnace.

After take-off the alumina medium is separated from the sintered product and maybe reused again in the furnace. The sintered product is conveyed to a convenient collecting station where it is allowed to cool.

OBJECTS OF THE INVENTION become apparent to one skilled in the art upon employment of the invention in practice.

DESCRIPTION OF THE DRAWINGS A preferred embodiment has been chosen for the purpose of illustrating and describing the principles of the invention and is illustrated in the accompanying drawing wherein:

FIG. 1 is a side elevation view illustrating the vertical sintering furnace of the present invention; and

FIG. 2 is a'section view taken along line 2-2 of FIG.

Referring now to the drawing, particularly to FIG. 1, the sintering furnace 10 comprises a preheat section 12, a vertically disposed high temperature furnace 14, and a take-off and separating section 16.

The preheat section 12 comprises an enclosed casing 18 supported at one end upon the high temperature furnace 14 and at its other end by a suitable support member 20. The preheat section includes an infeed chute 21 and a preheat conveyor 22 over which pressed metal products 24 are conveyed and heated prior to entering the high temperature furnace. The preheat concurtain to burn off any hydrogen which may escape through the opening in the preheat furnace.

The powdered metal products enter the preheat section on the infeed chute '21 and are deposited onto the preheat conveyor 22. The preheat conveyor delivers the pressed products onto an output chute 36 which feeds into the high temperature furnace 14.

The high temperature furnace 14 receives a mixture of the powdered metal parts 24 and a conveying medium 38 at a funnel 40 located at the top of the furnace. A vertical screw conveyor 41 delivers the conveying medium to the funnel 40 through a suitable chute 43. A

suitable motor'45 drives the screw conveyor.

The medium serves to separate the metal parts from each other and to convey them through the furnace for sintering. The medium is selected for its thermal stability and its ability to retain a free flowing character at the elevated temperatures. A fine mesh, for example between 50 and 100 mesh alumina sand is suitable for this purpose. Magnesium oxide in pebble grain form may also be used however it cannot withstand furnace temperatures as high as alumina sand can.

The mix of alumina medium and metal parts enters the high temperature furnace and passes through a central vertical tube 42 for sintering. A heating element 44, preferably an electric resistance heater wound in a spiral around the central tube, heats the metal parts to a desired temperature, for instance, to a selected temperature between l,500 F to 2,900 F in a continuous operation. The central vertical tube 42 is carried by a furnace casing 46 of suitable sheet steel. The interior of the casing is lined with refractory brick 48 for insulation. Loose insulation 50 may be used in the space 52 I immediately surrounding the tube.

If desired an electric induction heater may be used to heat the furnace in which case the alumina medium will act as an insulator. However, any suitable heating means may be used.

The high temperature furnace includes a lower support structure 54 which provides sufficient clearance to allow for take-off equipment 56 to be positioned under the furnace.

The vertical tube 42 passes through the bottom wall 58 of the furnace casing for delivering the conveying medium and the sintered product onto a take-off conveyor 60.

Flow of the alumina conveying medium and sintered product through the furnace, is controlled by means of the take-off conveyor 60 located at the bottom of the vertical tube. The alumina and the sintered product move under the force of gravity and the rate of flow increases as the linear speed of the conveyor increases. The take-off conveyor which is preferably a mesh belt removes the medium and product 24, carrying them over a funnel 62 which collects the alumina. The alumina is moved by the screw conveyor 41 to the top of the furnace for recycling.

The separation of the sintered product from the al'umina is accomplished by using the porous mesh belt 60 through which the alumina falls into a funnel 62 at the input end of the screw conveyor 41. The funnel 62 includes a flange plate 63 below the conveyor for confining the alumina while it is being moved to the funnel. The sintered product is removed from the take-off conveyor and delivered into a suitable receptacle 66. The mesh belt conveyor is spaced by suitable end rolls 68 and is driven by a power unit 70 through a suitable belt 72 and sprocket arrangement 74. The power unit may be controlled to vary the conveyor speed and hence the rate at which product and medium pass through the furnace.

Preferably, the entire take-off area 56 of the sintering furnace including the lower end of tube 42 is encased in a water-cooled jacket 76 to lower the temperature of the sintered product and to maintain the ambient temperature for operating personnel within tolerable limits. The water-cooled jacket is of suitable double wall construction and includes input and outlet pipes 78 and 80 for circulating cooling water through the cooling jacket.

It will be observed that hydrogen is introduced into the sintering furnace through an input pipe 82 located near the bottom of the vertical tube 42. The hydrogen percolates through the alumina into the high temperature furnace and into the preheat furnace. A suitable burn-off flame 84 is maintained for consuming excess hydrogen and also to indicate the presence of the protective hydrogen atmosphere within the furnace. The take-off area 56 of the furnace is enclosed to maintain hydrogen within the furnace. The water jacket is provided with end walls 86 and 88 for this purpose. The receptacle 66 includes a cover 90 connected to an outlet tube 92 for further enclosing the furnace. A suitable valve 94 closes tube 92 when removing the receptacle 66 to prevent hydrogen loss.

In operation and in carrying out the method of the invention the green pressed products 24 are introduced into the preheat area 12 of the sintering furnace and are heated to a selected temperature between 1,000 to l,500 F in a protective hydrogen atmosphere. The preheated product is conveyed to the input end of the high temperature zone 14. At this point the alumina 38 is mixed with the green pressed products 24 and to provide the medium by which the green pressed product is moved through the sintering furnace 14. The green pressed products are distributed through the medium out of contact with each other for proper sintering action.

Hydrogen is introduced into the lower end of the furnace and percolates through the mixture to provide the necessary protective atmosphere in which sintering occurs. Suitable arrangements are made at the preheat and take-off areas to consume whatever hydrogen escapes from the furnace.

The rate of flow through the furnace is controlled by a mesh-type belt take-off conveyor 60 which carries the alumina and sintered product away from the bottom end of the sintering furnace. In other words, by increasing the linear speed of the conveyor a greater volume of alumina and sintered product flow by gravity through the vertical shaft. Alumina which is of very fine consistency, preferably approximately mesh, falls through the mesh take-off conveyor into a suitable funnel 62 and thereafter is returned by a screw conveyor 41 to the input end of the furnace 40. The sintered product is dropped off the take-off conveyor into a suitable collecting barrel 66.

By using this arrangement this invention provides the highly advantageous possibility of sintering at temperatures up to 2,900 F. The higher temperature produces sintered products of improved quality and strength. The high temperature is confined to the central core area of the high temperature furnace and more specifically to the heating coil and to the vertical tube 42 passing through the furnace. There are no moving parts such as are presently used in horizontal conveyor furnaces which are destroyed by the high temperatures which the present invention is able to operate at. Consequently, the life expectancy of the furnace components is greatly increased.

The vertical tube 42 in the high temperature zone may easily be removed for inspection or replacement, in virtue of the space 52 between the refractory brick and the central vertical shaft. This space is filled with loose insulation.

What is claimed is:

1. A sintering furnace for pressed metal parts including a preheat section having an enclosed casing and a conveyor for receiving and moving parts through the enclosed casing, means for heating the pressed parts up to a temperature of about l,500F. on the conveyor, a high temperature section having an elongated conduit arranged with its inlet elevated with respect to the outlet to allow gravity flow through the conduit, a funnel fitted to the inlet of the conduit for receiving the parts from the preheat conveyor, means for delivering a carrying medium to the funnel so the parts are distributed in the medium upon entering the conduit, said medium being 50 to 100 mesh alumina sand, means for heating the parts in the conduit throughout more than half the entire length of said conduit up to approximately 2,900 F., said heating means being disposed about more than half the entire length of said conduit, said heating means being an electrical resistance heater wound in a spiral around said conduit, said heater being surrounded by insulating brick, a conveyor located beneath the outlet from the conduit to receive the discharge of the medium and parts, and means to vary the linear speed of the outlet conveyor to control the rate of gravity flow of the medium and parts through the conduit, said outlet conveyor being adapted to separate the medium from the parts, and means for cooling the parts on the outlet conveyor.

2. The furnace of claim 1 wherein the cooling means comprises a water-cooled jacket surrounding the outlet conveyor.

3. The furnace of claim 1 which further includes means for introducing a protective atmosphere into the furnace.

4. The furnace of claim 1 which further includes means; for returning the medium to input end of the conduit. 

2. The furnace of claim 1 wherein the cooling means comprises a water-cooled jacket surrounding the outlet conveyor.
 3. The furnace of claim 1 which further includes means for introducing a protective atMosphere into the furnace.
 4. The furnace of claim 1 which further includes means for returning the medium to input end of the conduit. 